How to render a world position pass that includes volumetrics?
Looks like (with the help of @iponoo @zeauro &amp; @ kologe from blender artists &amp; blenderchat), it is possible.
Here's the smoke shader, that works with the mesh shader to produce a WPP for both smoke and meshes-


It works well enough for me to do a bit of 'deep' type comping-
https://youtu.be/xgCA-D4ukJk

In the video the smoke seems to be respecting the same XYZ-RGB space as the meshes.
So divide the vectors by 100 in the smoke shader seems to make things all work together.

And certainly work well enough with the 'volume mask' tool in Fusion for me.

Cheers!